AC bridges (rectifiers) are used for rectification of three-phase or AC automotive generators (dynamos). Semiconductor diodes having a pn junction of silicon are generally used as rectifying elements. For example, six semiconductor diodes are wired together to form a B6 bridge in a three-phase generator. Diodes are occasionally also used in parallel circuits, using twelve diodes instead of six, for example. Suitably adapted diode bridges are also used in AC generators having a different number of phases.
These diodes are designed for operation at high currents or current densities of more than 500 A/cm2 and high temperatures or a maximum barrier layer temperature Tj of approximately 225° C. The voltage drop in the forward direction, i.e., forward voltage UF, typically amounts to about 1 volt at the high currents used. Generally only a very low reverse current IR flows during operation in the reverse direction below breakdown voltage UZ. The reverse current increases drastically beyond breakdown voltage UZ. A further increase in voltage is therefore prevented. Z diodes having reverse voltages of approximately 20-50 volts, depending on the vehicle electrical system voltage of the particular motor vehicle, are used in most cases. Z diodes are capable of carrying high currents briefly in a breakdown. They are therefore used to limit the overshooting generator voltage in load changes (load dump). Such diodes are generally packaged in sturdy press-fit diode housings such as those described in DE 195 49 202 B4, for example.
The forward voltage of pn diodes results in forward power losses and thus has a negative effect on the efficiency of the generator. On the average, two diodes are always connected in series in current output by the generator, so the average forward power losses with a 100 A generator amount to approximately 200 W. These losses result in heating of the diodes. The resulting heat must be dissipated to the ambient air or cooling air through complex cooling measures involving the rectifier, for example, using cooling elements and/or a fan.
German Published Patent Application No. 10 2004 056 663 describes the use of so-called high-efficiency Schottky diodes (HED) instead of pn diodes to reduce the forward power losses. High-efficiency Schottky diodes are diodes having a low forward voltage and a reverse current almost independent of the reverse voltage.
German Published Patent Application No. 10 2004 053 760 describes a trench MOS barrier Schottky diode, including an integrated pn diode, as one possible exemplary embodiment of an HED Schottky diode. Such a diode is also known as a TJBS-PN. In addition to the low forward voltage in the conducting state, this also limits the overshooting generator voltage, which may occur with sudden load changes, to uncritical values, typically to voltages of less than 30 V in 14 V systems.
Much lower forward voltages UF in the range of 0.5 V to 0.7 V are achievable using high-efficiency Schottky diodes. The efficiency and power output of the generator are increased due to the low forward power losses of these diodes. The expenditure for cooling may also be reduced substantially in comparison with the use of pn diodes due to the lower power losses.
Due to the breakdown voltage of an HED, the generator voltage, which rises when a load dump occurs, is limited. High electric powers are converted into heat on the diode for a short period of time, typically less than a few 100 milliseconds. In the case of a TJBS-PN, the integrated pn structures act as voltage-limiting Z diodes. The pn structures are operated in avalanche breakdown. The power drop at the diode corresponds to the product of the reverse voltage of the diode and the generator current. Due to the high power loss, the diode heats up to very high temperatures during this process. Barrier layer temperatures or junction temperatures Tj of more than 225° C. may occur. Since the avalanche breakdown voltage VZ increases with the temperature, the voltage actually occurring during the load dump is a few volts higher than reverse voltages VZ measured at low current densities and at room temperature. As a result, in the event of a breakdown, voltages of more than 30 volts may occur briefly in a 14 V vehicle electrical system. A simple drop in breakdown voltage VZ finds its limit in the voltage ripple of the generators (ripple). In modern vehicle electrical system architectures, there is a growing trend toward limiting the maximum occurring vehicle electrical system voltage to low levels, for example, to 27 V, in the event of a malfunction.